Therefore, Mark Twain could have read "Anne of Green Gables" if he lived long enough after its publication.
It's worth noting that Twain was 75 years old when the book came out, and while he lived until 1910, his health was declining in his last years. It's unlikely he would have been reading a lot of new books, particularly children's literature, during that time.
However, we can't say for sure that he never read it. It's possible he saw it, read a review, or perhaps even someone read it to him.